This is a collection of extracts from Schrödinger's 1944 book
What is life? with some comments added by me, mainly trying to clarify
what I think he is saying and why I think it is so important for anyone trying
to understand how life as we know it is possible in this physical universe. What
needs to be explained is not just how particular forms of life are possible, but
how the mechanisms of physics and chemistry support the continual production of
new forms of life. It is often claimed that the theory of natural selection
answers that question, but it doesn't answer the question: what provides the
increasingly complex and varied collections of options for natural selection to
choose from? My (partial) answer is: an increasingly complex and varied
collection of construction kits, themselves products of evolution by natural
selection. (Compare recursive structure-building functions in computing.)

What Schrödinger tried to do in this book could be
described as attempting to characterise key features of the
Fundamental Construction Kit (FCK) required to support the many types of
Derived Construction Kit (DCK) required for biological evolution to have
produced the many and varied forms of life we now have on our planet, as
discussed in
http://www.cs.bham.ac.uk/research/projects/cogaff/misc/construction-kits.html

What is Life?

Late in 2015, while working on a paper on evolution's use of construction
kits of many kinds
Sloman(2016)
I re-read Schrödinger's little book,
Erwin Schrödinger,
What is life?, CUP, Cambridge, 1944,
for the first time for many years, and was surprised to find that he had
presented so many of the crucial ideas required to explain the possibility of
biological evolution and the possibility of construction kits required for
development and reproduction.

There are several problems. First the second law of thermodynamics states that
complex systems will become increasingly disordered, whereas the opposite is
true of individual living things and life in general, which become increasingly
complex and ordered as a result of biological evolution. Part of the explanation
is very familiar: since neither the earth, nor any individual organism on the
earth is an isolated physical system, external sources of energy, including
solar energy, heat energy from the earth's core, and chemical energy can counter
the tendency to increased entropy for some entities on the planet.

But that does not explain the origins or persistence of increasingly complex and
detailed structures. that are required to preserve biological information during
the development of an organism, and during reproductive processes across many
generations.

The basic problem is how the genetic information is preserved both
within an individual during the complexities of development and across
individuals over generations. A secondary problem is how so much new complexity
evolves over time. We'll first focus on the answer Schrödinger [ES] gives
to the question how it is possible for detailed specifications, encoded in
complex molecules to survive across generations despite constant thermal
buffetting and potentially disruptive influences during development and
reproduction, despite the second law of thermodynamics and despite the fact that
the fundamental mechanisms of quantum physics are statistical.

ES provides an answer by pointing out that although quantum theory implies that
physical processes essentially involve statistical patterns of change and are
therefore not deterministic, it also implies that there can be structures that
are in stable states because, although they are capable of switching to new
states, they are very unlikely to do so, unless affected by a sufficiently high
energy impulse. That allows quantum mechanics to support not only indeterminism,
but also long term determinism.

Moreover if a physical structure is in stable state S1 it may be capable of
having another stable state S2, which may be at either a higher or a lower level
energy state, or the same level as the original state.

The states are stable because the transition from one to the other, or from one
of them to some other state requires a minimum energy packet to "get over a
hump".

Many human-designed mechanisms use this feature, for example, a box with a
liftable hinged lid that is stable when shut and also when
opened and folded back, like some dustbin lids. In that case gravity provides
the force that has to be overcome to move the lid from one stable state to
another. Another familiar example is the commonly used lever and spring
mechanism that forces a wall-mounted electric light switch to
be in one or other of two stable positions, using the
"toggle" design
https://en.wikipedia.org/wiki/Light_switch#Toggle.
Many engineering designs depend on multi-stability, including the combination of
springs and levers that allow a heavy hinged item, such as a car boot (trunk)
lid to be held safely (i.e. stably) in more than one position, i.e. when shut or
when fully open. As it moves up or down springs are stretched or released, and
when stretched they hold potential energy. When the lid is fully open gravity
alone does not suffice to pull it down past the maximum energy peak.

In some cases, instead of two or more fixed stable states a mechanism can allow
collections of states each of which allows free motion, whereas the transition
from one state to the other requires energy. An example is a flat tray that has
grooves and circular hollows on it, and a marble that can move horizontally on
the tray, while kept on the tray by the earth's gravitational pull. If the
marble falls into one of the hollows or grooves it will be able to move around
freely in the hollow or groove, but will not be able to jump to one of the other
depressed parts of the tray unless the tray is given an extra strong jolt that
communicates an impulse to the marble, making it jump out of the groove, and
allowing it to move to other parts of the tray. How long a marble will resist
being jolted out of a groove will depend on how deep the groove is, and how
powerful the jolts are.

The above examples are partly analogous to the situation ES describes in which a
molecule composed of several atoms may have two stable states which differ only
in the location of one of the atoms.

Figure Isomeres

Two molecules with the same types of atoms connected differently
Each may be stable in the absence of a disruptive external influence

E.g. the two isomeres of propyl alcohol differ only in whether the oxygen atom
(the blue "O" in the figure) is bound to the central carbon atom or an end
carbon atom. Each state is stable because all their neighbouring states require
higher energy. But if a sufficiently energetic impulse is received it can push
the molecule over the energy "hump" and into the other stable state. This
example is used in section 39 of the book, as the basis of several deep
observations relevant to biological evolution.

In Chapter 7, ES discusses additional questions about the increasing complexity
and variety of products of evolution and how that can be reconciled with what we
know about the physical universe.

THE FORMAT OF THIS DOCUMENT

My comments from here on will be indented and italicised, as in this section,
whereas quotations from the book are not indented and not italicised.

Many detailed technical sections of the text, and all mathematical sections, are
omitted, in order to make this easy for a non-expert to read. I have also
occasionally inserted paragraph breaks to help the reader.

After drawing attention to some biological phenomena and a background of
physical laws, ES summarises puzzling biological phenomena that he wishes to
show can be understood in the framework of Quantum mechanics but not previous
physical theories (e.g. Newtonian mechanics augmented by statistical mechanics).

By the time the book was published (1944) there was already evidence that
biological genetic information was stored and transmitted in extended chemical
structures, and it was assumed that parts of those structures could specify
particular inherited features. ES emphasises the fact that in some cases of
biological inheritance, a particular unusual feature, which may be a product of
a small portion of the genetic material can persist across several generations.
He takes the "Habsburg lip" as an example. The reliable transition of a special
feature across several reproductive episodes, each involving the development of
a whole human from a fertilized egg cries out for explanation, as would
preservation and replication of a triangular shape drawn in sand across Saharan
sand dunes.

I think it is fair to say that the latter is impossible. ES tries to show what's
special about genetic material that makes reproduction and preservation of
detailed structure possible across even more complex disruptive processes than
sand-storms. But he also tries to bring out why that is such a remarkable
achievement and why it would have been impossible to explain on the basis of
pre-quantum physics. For example, life as we know it would not have been
possible in a universe composed of Newtonian point masses with mutual
gravitational attraction. (I think Newton noticed this limitation of "Newtonian"
mechanics, but I am not a Newton-scholar.)

ANNOTATED EXTRACTS FROM WHAT IS LIFE?

Chapter IV
THE QUANTUM-MECHANICAL EVIDENCE
32. Permanence unexplainable by classical physics....
We are now seriously faced with the question: How can we, from the
point of view of statistical physics, reconcile the facts that the
gene structure seems to involve only a comparatively small number
of atoms (of the order of 1,000 and possibly much less), and that
value nevertheless it displays a most regular and lawful activity -
with a durability or permanence that borders upon the miraculous?

ES uses the 'Habsburg Lip' as an example:

Fixing our attention on the portraits of a member of the family in
the sixteenth century and of his descendant, living in the
nineteenth, we may safely assume that the material gene structure,
responsible for the abnormal feature, has been carried on from
generation to generation through the centuries, faithfully
reproduced at every one of the not very numerous cell divisions
that lie between. Moreover, the number of atoms involved in the
responsible gene structure is likely to be of the same order of
magnitude as in the cases tested by X-rays. The gene has been kept
at a temperature around 98°F during all that time. How are we to
understand that it has remained unperturbed by the disordering
tendency of the heat motion for centuries?
....
Of the existence, and sometimes very high stability, of these
associations of atoms, chemistry had already acquired a widespread
knowledge at the time. But the knowledge was purely empirical. The
nature of a molecule was not understood - the strong mutual bond of
the atoms which keeps a molecule in shape was a complete conundrum
to everybody.
....
The evidence that two features, similar in appearance, are based on
the same principle, is always precarious as long as the principle
itself is unknown.

These extracts from the book indicate why such phenomena are problematic for
current theories of physics, including thermodynamics, and chemistry. In all the
statistical flux of matter in motion at temperatures of human bodies, how could
something as minute as a molecular fragment specifying some biological feature,
survive unchanged, even across many generations, despite all the copying
required for reproduction and development? An outline answer follows:

33. Explicable by quantum theory
In this case it is supplied by quantum theory. In the light of
present knowledge, the mechanism of heredity is closely related to,
nay, founded on, the very basis of quantum theory.
....
The Heitler-London theory involves the most subtle and intricate
conceptions of the latest development of quantum theory (called
'quantum mechanics' or 'wave mechanics').
....
34. Quantum theory--discrete states--quantum jumps
The great revelation of quantum theory was that features of discreteness
were discovered in the Book of Nature, in a context in which
anything other than continuity seemed to be absurd according to the
views held until then.
....
For small-scale systems most of these or similar characteristics
--we cannot enter into details-- change discontinuously. They are
'quantized', just as the energy is. The result is that a number of
atomic nuclei, including their bodyguards of electrons, when they
find themselves close to each other, forming 'a system', are unable
by their very nature to adopt any arbitrary configuration we might
think of. Their very nature leaves them only a very numerous but
discrete series of 'states' to choose from. We usually call them
levels or energy levels, because the energy is a very relevant part
of the characteristic. But it must be understood that the complete
description includes much more than just the energy. It is
virtually correct to think of a state as meaning a definite
configuration of all the corpuscles. The transition from one of
these configurations to another is a quantum jump.
....

35. Molecules
Among the discrete set of states of a given selection of atoms there
need not necessarily but there may be a lowest level, implying a close
approach of the nuclei to each other. Atoms in such a state form a
molecule. The point to stress here is, that the molecule will of
necessity have a certain stability; the configuration cannot change,
unless at least the energy difference, necessary to 'lift' it to the
next higher level, is supplied from
outside.

NOTE:
Some of the mathematical details in the book are skipped here. It turns out that
there can be two possible states of a molecule with the same or similar energy
levels, between which there are only transitions requiring much higher energy
levels -- as in the toggle switch and car boot lid examples above.
So either state could be equally stable at a given temperature. In other
words, just because two states of molecule have the same energy it does not
follow (in quantum physics) that it is easy to switch the molecule between those
two states.

38. First amendment
In offering these considerations as a theory of the stability of
the molecule it has been tacitly assumed that the quantum jump
which we called the 'lift' leads, if not to a complete
disintegration, at least to an essentially different configuration
of the same atoms -- an isomeric molecule, as the chemist would say,
that is, a molecule composed of the same atoms in a different
arrangement (in the application to biology it is going to represent
a different 'allele' in the same 'locus' and the quantum jump will
represent a mutation).
To allow of this interpretation two points must be amended in our story, which I
purposely simplified to make it at all intelligible.
From the way I told it, it might be imagined that only in its very
lowest state does our group of atoms form what we call a molecule
and that already the next higher state is 'something else'. That is
not so. Actually the lowest level is followed by a crowded series
of levels which do not involve any appreciable change in the
configuration as a whole, but only correspond to those small
vibrations among the atoms which we have mentioned in §37.
....
So the first amendment is not very serious: we have to disregard
the 'vibrational fine-structure' of the level scheme. The term
'next higher level' has to be understood as meaning the next level
that corresponds to a relevant change of configuration.

39. Second amendment
The second amendment is far more difficult to explain,
because it is concerned with certain vital, but rather complicated,
features of the scheme of relevantly different levels.

The
free passage between two of them may be obstructed, quite apart
from the required energy supply; in fact, it may be obstructed even
from the higher to the lower state.
....
It is known to the chemist that
the same group of atoms can unite in more than
one way to form a molecule. Such molecules are
called isomeric ('consisting of the same parts').
....
Isomerism is not an exception, it is the rule. The
larger the molecule, the more isomeric
alternatives are offered.

Isomerism is illustrated in the figure above, copied
from the book. The two molecules have the same constituents, but because the
oxygen atom has different locations in the two molecules they have very
different physical and chemical properties. And neither state can easily be
transformed into the other because the transition between the two states
requires the molecule to pass through intermediate configurations which have a
greater energy than either of them. ES writes:

The remarkable fact is that both molecules are perfectly stable, both behave as
though they were 'lowest states'. There are no spontaneous transitions from
either state towards the other.

The reason is that the two configurations are not neighbouring configurations.
The transition from one to the other can only take place over intermediate
configurations which have a greater energy than either of them. To put it
crudely, the oxygen has to be extracted from one position and has to be inserted
into the other. There does not seem to be a way of doing that without passing
through configurations of considerably higher energy.
....
Now we can give our 'second amendment', which is that transitions
of this 'isomeric' kind are the only ones in which we shall be
interested in our biological application. It was these we had in
mind when explaining 'stability' in §§35-37
....

Chapter V
DELBRÜCK'S MODEL DISCUSSED AND TESTED

40. The general picture of the hereditary substance
From these facts emerges a very simple answer to our question,
namely: Are these structures, composed of comparatively few atoms,
capable of withstanding for long periods the disturbing influence
of heat motion to which the hereditary substance is continually
exposed?
We shall assume the structure of a gene to be that of a huge molecule, capable
only of discontinuous change, which consists in a rearrangement of the atoms and
leads to an isomeric molecule. The rearrangement may affect only a small region
of the gene, and a vast number of different rearrangements may be possible.
The energy thresholds, separating the actual configuration from any
possible isomeric ones, have to be big enough (compared with the
average heat energy of an atom) to make the change-over a rare
event. These rare events we shall identify with spontaneous
mutations.
....

Note: this was published several years before the discovery of the
"Double Helix" structure of DNA by Watson, Crick and their collaborators.

41. The uniqueness of the picture
Was it absolutely essential for the biological question to dig up
the deepest roots and found the picture on quantum mechanics? The
conjecture that a gene is a molecule is today, I dare say, a
commonplace. Few biologists, whether familiar with quantum theory
or not, would disagree with it.
....
Why did I so strongly insist on the quantum-mechanical point of
view, though I could not really make it clear in this little book
and may well have bored many a reader?

Quantum mechanics is the first theoretical aspect which accounts
from first principles for all kinds of aggregates of atoms actually
encountered in Nature. The Heitler-London bondage is a unique,
singular feature of the theory, not invented for the purpose of
explaining the chemical bond. It comes in quite by itself, in a
highly interesting and puzzling manner, being forced upon us by
entirely different considerations.
.....
Consequently, we may safely assert that there is no alternative to the molecular
explanation of the hereditary substance. The physical aspect leaves no other
possibility to account for itself and of its permanence.
.....

42. Some traditional misconceptions

NOTE;
This section discusses possible questions and confusions about similarities and
differences between solids (crystalline and amorphous), liquids, gases, and
which sorts of material can resist change of structure over long periods of
time.

43. Different states of matter
Now I would not go so far as to say that all these statements and distinctions
are quite wrong. For practical purposes they are sometimes useful. But in the
true aspect of the structure of matter the limits must be drawn in an entirely
different way. The fundamental distinction is between the two lines of the
following scheme of 'equations':

molecule = solid = crystal.
gas = liquid = amorphous.

We must explain these statements briefly. The so-called amorphous
solids are either not really amorphous or not really solid. In
'amorphous' charcoal fibre the rudimentary structure of the
graphite crystal has been disclosed by X-rays. So charcoal is a
solid, but also crystalline. Where we find no crystalline structure
we have to regard the thing as a liquid with very high 'viscosity'
(internal friction). Such a substance discloses by the absence of a
well-defined melting temperature and of a latent heat of melting
that it is not a true solid.

.... further details omitted here ....

44. The distinction that really matters....
The distinction that is really important in the structure of small
matter is whether atoms are bound together by those Heitler-London
forces or whether they are not. In a solid and in a molecule they
all are. In a gas of single atoms (as e.g. mercury vapour)
they are not. In a gas composed of molecules, only the atoms within
every molecule are linked in this way.

45. The aperiodic solid
A small molecule might be called 'the germ of a solid'. Starting
from such a small solid germ, there seem to be two different ways
of building up larger and larger associations. One is the
comparatively dull way of repeating the same structure in three
directions again and again. That is the way followed in a growing
crystal.
....
The other way is that of building up a more and more extended aggregate
without the dull device of repetition. That is the case of the more and
more complicated organic molecule in which every atom, and every
group of atoms, plays an individual role, not entirely equivalent
to that of many others (as is the case in a periodic structure). We
might quite properly call that an aperiodic crystal or solid and
express our hypothesis by saying: We believe a gene --or perhaps the
whole chromosome fibre --to be an aperiodic solid.
....

NOTE:
In the next section ES shows that he understood the requirement
for diversity in the genetic code and that this requirement can be met by his
proposed molecular encoding mechanism, whose diversity of possible encodings
increases exponentially with the length of the code. This was published a few
years before Shannon (1948), but seems to have
anticipated some of the ideas about requirements for transmission and storage of
information.

46. The variety of contents compressed in the miniature code
It has often been asked how this tiny speck of material, nucleus of
the fertilized egg, could contain an elaborate code-script
involving all the future development of the organism.
....
Indeed, the number of atoms in such a structure need not be very
large to produce an almost unlimited number of possible
arrangements. For illustration, think of the Morse code. The two
different signs of dot and dash in well-ordered groups of not more
than four allow thirty different specifications. Now, if you
allowed yourself the use of a third sign, in addition to dot and
dash, and used groups of not more than ten, you could form 88,572
different 'letters'; with five signs and groups up to 25, the
number is 372,529,029,846,191,405.

NOTE:
I have checked his calculations in these two examples! (A.S.)

....
What we wish to illustrate is simply that with the molecular picture of the gene
it is no longer inconceivable that the miniature code should precisely
correspond with a highly complicated and specified plan of development and
should somehow contain the means to put it into operation.

47. Comparison with facts: degree of stability; discontinuity of
mutations....
Thus the threshold values the chemist encounters are of necessity precisely of
the order of magnitude required to account for practically any degree of
permanence the biologist may encounter; for we recall from §36
that thresholds
varying within a range of about 1:2 will account for lifetimes ranging from a
fraction of a second to tens of thousands of years.
....
These considerations make it conceivable that an isomeric change of
configuration in some part of our molecule is, produced by a chance fluctuation
of the vibrational energy, can actually be a sufficiently rare event to be
interpreted as a spontaneous mutation. Thus we account, by the very principles
of quantum mechanics, for the most amazing fact about mutations, the fact by
which they first attracted de Vrie's attention, namely, that they are 'jumping'
variations, no intermediate forms occurring.

48. Stability of naturally selected genes....
Granted that we have to account for the rare natural mutations by
chance fluctuations of the heat motion, we must not be very much
astonished that Nature has succeeded in making such a subtle choice
of threshold values as is necessary to make mutation rare. For we
have, earlier in these lectures, arrived at the conclusion that
frequent mutations are detrimental to evolution.
Individuals which, by mutation, acquire a gene configuration of
insufficient stability, will have little chance of seeing their
'ultra-radical', rapidly mutating descendancy survive long. The
species will be freed of them and will thus collect stable genes by
natural selection.

49. The sometimes lower stability of mutants

NOTE:
In this and the next section ES points out that whereas it is important for the
majority of the genetic material to be highly stable, there must be some
instability for mutations to occur. Moreover molecular instability can increase
if temperature is increased. But if mutant genes are already unstable, a
temperature increase should have a smaller effect on them than on more stable
non-mutant genes. I've omitted most of the details.

But, of course, as regards the mutants which occur in our breeding
experiments and which we select, qua mutants, for studying their
offspring, there is no reason to expect that they should all show
that very high stability. For they have not yet been 'tried out'
--or, if they have, they have been 'rejected' in the wild breeds
--possibly for too high mutability. At any rate, we are not at all
astonished to learn that actually some of these mutants do show a
much higher mutability than the normal 'wild' genes.

50. Temperature influences unstable genes less than stable ones....
The time of expectation is diminished by raising the temperature, the mutability
is increased. Now that can be tested and has been tested with the fly Drosophila
in the range of temperature which the insects will stand. The result was, at
first sight, surprising. The low mutability of wild genes was distinctly
increased, but the comparatively high mutability occurring with some of the
already mutated genes was not, or at any rate was much less, increased. That is
just what we expect on comparing our two formulae.
....

NOTE
The predicted effects of Xrays are different from predictions for temperature
increases. The effects of Xrays on the molecules they affect are more
"explosive" (via production of ionised particles) and might be expected to
affect normal and mutant genes in similar ways. Predicted effects are observed,
helping to support the theory being presented. Details are omitted here.

51. How x-rays produce mutation....52. Their efficiency does not depend on spontaneous mutability....53. Reversible mutations

NOTE
Some mutations are reversible. One might expect the energy for the original
mutation and for the reverse mutation to be the same. But not if the mutated
molecule and the original molecule have different energy levels, with a high
energy barrier separating them. In that case the mutation from the higher energy
molecule to the lower energy molecule might occur more frequently than the
reverse mutation, since the reverse change requires a "bigger kick" to get over
the hump. (My paraphrase.) Observed differences in rates of mutation in
opposite directions are consistent with this theory.

Chapter VI:
ORDER, DISORDER AND ENTROPY

54. A remarkable general conclusion from the model
Let me refer to the phrase on p. 62, in which I
tried to explain that the molecular picture of the
gene made it at least conceivable that the
miniature code should be in one-to-one
correspondence with a highly complicated and
specified plan of development and should
somehow contain the means of putting it into
operation.

Very well then, but how does it do
this? How are we going to turn 'conceivability'
into true understanding? Delbrück's molecular
model, in its complete generality, seems to
contain no hint as to how the hereditary
substance works,
Indeed, I do not expect that any
detailed information on this question is likely to
come from physics in the near may future. The
advance is proceeding and will, I am sure,
continue to do so, from biochemistry under the
guidance of physiology and genetics.

No detailed information about the functioning of the genetical mechanism can
emerge from a description of its structure so general as has been given above.
That is obvious. But, strangely enough, there is just one general conclusion to
be obtained from it, and that, I confess, was my only motive for writing this
book. From Delbruck's general picture of the hereditary substance it emerges
that living matter, while not eluding the 'laws of physics' as established
up to date, is likely to involve 'other laws of
physics' hitherto unknown, which, however, once
they have been revealed, will form just as
integral a part of this science as the former.

55. Order based on order
This is a rather subtle line of thought, open to
misconception in more than one respect. All the
remaining pages are concerned with making it
clear. A preliminary insight, rough but not
altogether erroneous, may be found in the
following considerations:

It has been explained
in chapter 1 that the laws of physics, as we know
them, are statistical laws. They have a lot to do
with the natural tendency of things to go over
into disorder.

But, to reconcile the high
durability of the hereditary substance with its
minute size, we had to evade the tendency to
disorder by 'inventing the molecule', in fact, an
unusually large molecule which has to be a
masterpiece of highly differentiated order,
safeguarded by the conjuring rod of quantum
theory.

The laws of chance are not invalidated by
this 'invention', but their outcome is modified.
The physicist is familiar with the fact that the
classical laws of physics are modified by
quantum theory, especially at low
temperature.

There are many instances of this.
Life seems to be one of them, a particularly
striking one. Life seems to be orderly and lawful
behaviour of matter, not based exclusively on its
tendency to go over from order to disorder, but
based partly on existing order that is kept up.

To the physicist --but only to him-- I could hope to
make my view clearer by saying: The living
organism seems to be a macroscopic system
which in part of its behaviour approaches to that
purely mechanical (as contrasted with
thermodynamical) conduct to which all systems
tend, as the temperature approaches absolute
zero and the molecular disorder is removed.

The non-physicist finds it hard to believe that really
the ordinary laws of physics, which he regards as
the prototype of a part inviolable precision,
should be based on the statistical tendency of
matter to go over into disorder. I have given
examples in Chapter 1. The general principle
involved is the famous Second Law of
Thermodynamics (entropy principle) and its
equally famous statistical foundation.

In §§56-60 I will try to sketch the bearing of the entropy
principle on the large-scale behaviour of a living
organism --forgetting at the moment all that is
known about chromosomes, inheritance, and so
on.

56. Living matter evades the decay to equilibrium
What is the characteristic feature of life? When is a piece of matter said to be
alive? When it goes on 'doing something', moving, exchanging material with its
environment, and so forth, and that for a much longer period than we would
expect of an inanimate piece of matter to 'keep going' under similar
circumstances. When a system that is not alive is isolated or placed in a
uniform environment, all motion usually comes to a standstill very soon as a
result of various kinds of friction; differences of electric or chemical
potential are equalized, substances which tend to form a chemical compound do
so, temperature becomes uniform by heat conduction.

After that the whole system fades away into a dead, inert lump of matter. A
permanent state is reached, in which no observable events occur. The physicist
calls this the state of thermodynamical equilibrium, or of `maximum entropy'.
Practically, a state of this kind is usually reached very rapidly.
....
These ultimate slow approaches to equilibrium could never be
mistaken for life, and we may disregard them here. I have referred
to them in order to clear myself of a charge of Inaccuracy.

57. It feeds on 'negative entropy'
It is by avoiding the rapid decay into the inert state of
'equilibrium' that an organism appears so enigmatic; so much so,
that from the earliest times of human thought some special
non-physical or supernatural force (vis viva, entelechy) was
claimed to be operative in the organism, and in some quarters is
still claimed. How does the living organism avoid decay?
The obvious answer is: By eating, drinking, breathing
and (in the case of plants) assimilating. The
technical term is metabolism.
....
For a while in the past our curiosity was silenced by being told
that we feed upon energy.
....
Needless to say, taken literally, this is just as absurd. For an adult organism
the energy content is as stationary as the material content.
....
What then is that precious something contained in our food which keeps us from
death? That is easily answered. Every process, event, happening -- call it what
you will; in a word, everything that is going on in Nature means an increase of
the entropy of the part of the world where it is going on. Thus a living
organism continually increases its entropy -- or, as you may say, produces
positive entropy -- and thus tends to approach the dangerous state of maximum
entropy, which is of death. It can only keep aloof from it, i.e. alive, by
continually drawing from its environment negative entropy -- which is something
very positive as we shall immediately see. What an organism feeds upon is
negative entropy. Or, to put it less paradoxically, the essential thing in
metabolism is that the organism succeeds in freeing itself from all the entropy
it cannot help producing while alive.

58. What is entropy?
Let me first emphasize that it is not a hazy
concept or idea, but a measurable physical
quantity just like of the length of a rod, the
temperature at any point of a body, the heat of
fusion of a given crystal or the specific heat of
any given substance.
....59. The statistical meaning of entropy
I have mentioned this technical definition simply
in order to remove entropy from the atmosphere
of hazy mystery that frequently veils it. Much
more important for us here is the bearing on the
statistical concept of order and disorder, a
connection that was revealed by the
investigations of Boltzmann and Gibbs in
statistical physics.
....
An isolated system or a system in a uniform environment (which for the present
consideration we do best to include as the part of the system we contemplate)
increases its entropy and more or less rapidly approaches the inert state of
maximum entropy. We now recognize this fundamental law of physics to be just the
natural tendency of things to approach the chaotic state (the same tendency that
the books of a library or the piles of papers and manuscripts on a writing desk
display) unless we obviate it. (The analogue of irregular heat motion, in this
case, is our handling those objects now and again without troubling to put them
back in their proper places.)

60. Organization maintained by extracting 'order' from the environment
How would we express in terms of the statistical
theory the marvellous faculty of a living
organism, by which it delays the decay into
thermodynamical equilibrium (death)? We said
before: 'It feeds upon negative entropy',
attracting, as it were, a stream of negative
entropy upon itself, to compensate the entropy
increase it produces by living and thus to
maintain itself on a stationary and fairly low
entropy level.
....
Thus the device by which an organism
maintains itself stationary at a fairly high level of
orderliness ( = fairly low level of entropy)
really consists continually sucking orderliness
from its environment.

This conclusion is less
paradoxical than it appears at first sight. Rather
could it be blamed for triviality. Indeed, in the
case of higher animals we know the kind of
orderliness they feed upon well enough, viz. the
extremely well-ordered state of matter in more or
less complicated organic compounds, which
serve them as foodstuffs. After utilizing it they
return it in a very much degraded form -- not
entirely degraded, however, for plants can still
make use of it. (These, of course, have their most
powerful supply of 'negative entropy' the sunlight.)

Sections 61--69 omitted

NOTE TO CHAPTER VI
(Included in 1955 edition)

The remarks on negative entropy have met with doubt and opposition
from physicist colleagues. Let me say first, that if I had been catering
for them alone I should have let the discussion turn on free energy
instead. It is the more familiar notion in this context. But this highly
technical term seemed linguistically too near to energy for making the
average reader alive to the contrast between the two things. He is
likely to take free as more or less an "epitheton ornans"[*] without much
relevance, While actually the concept is a rather intricate one, whose
relation to Boltzmann's order-disorder principle is less easy to trace than
for entropy and "entropy taken with a negative sign", which by the way
is not my invention. It happens to be precisely the thing on which
Boltzmann's original argument turned.
[*] "Decorative epithet"

But F. Simon has very pertinently pointed out to me that my simple
thermodynamical considerations cannot account for our having to feed
on matter "in the extremely well ordered state of more or less complicated
organic compounds" rather than on charcoal or diamond pulp. He is
right. But to the lay reader I must explain, that a piece of un-burnt
coal or diamond, together with the amount of oxygen needed for its
combustion, is also in an extremely Well ordered state, as the physicist
understands it. Witness to this: if you allow the reaction, the burning of
the coal, to take place, a great amount of heat is produced. By giving it
off to the surroundings, the system disposes of the very considerable
entropy increase entailed by the reaction, and reaches a state in which
it has, in point of fact, roughly the same entropy as before.

Yet we could not feed on the carbon dioxide that results from the
reaction. And so Simon is quite right in pointing out to me, as he did,
that actually the energy content of our food does matter; so my
mocking at the menu cards that indicate it was out of place. Energy is
needed to replace not only the mechanical energy of our bodily exertions,
but also the heat we continually give off to the environment. And that
we give off heat is not accidental, but essential. For this is precisely the
manner in which we dispose of the surplus entropy we continually
produce in our physical life process.

This seems to suggest that the higher temperature of the warm-blooded
animal includes the advantage of enabling it to get rid of its
entropy at a quicker rate, so that it can afford a more intense life process.
I am not sure how much truth there is in this argument (for which I am
responsible, not Simon). One may hold against it, that on the other
hand many warm-blooders are protected against the rapid loss of heat by
coats of fur or feathers. So the parallelism between body temperature
and "intensity of life", which I believe to exist, may have to be accounted
for more directly by van 't Hoff's law, mentioned at the end of Sect. 50[*]:
the higher temperature itself speeds up the chemical reactions involved
in living. (That it actually does, has been confirmed experimentally in
species which take the temperature of the surrounding.)
[*] Not yet included in this online document.

Chapter VII:
Is Life Based on the Laws of Physics?

61. New laws to be expected in the organism
("If a man never contradicts
himself, the reason must be that he virtually
never says anything at all."
Miguel De Unamuno (Quoted from conversation))

What I wish to make clear in this last chapter is,
in short, that from all we have learnt about the
structure of living matter, we must be prepared to
find it working in a manner that cannot be
reduced to the ordinary laws of physics. And that
not on the ground that there is any "new force" or
what not, directing the behaviour of the single
atoms within a living organism, but because the
construction is different from anything we have
yet tested in the physical laboratory.

NOTE
ES wrote this before the development, later in the 20th century, of computers
running complex interacting virtual machines, whose construction could rightly
be said to be different from anything physicists and engineers had previously
built or tested in physical laboratories, and whose properties and behaviours
cannot be described adequately in the language of the physical sciences, a point
that is elaborated in a separate document:

I suspect that if ES had been able somehow to spend a week or a month talking to
sophisticated AI researchers and software engineers in the 21st Century,
about the variety of types of virtual machinery that can run and interact on a
physical platform (or collection of connected physical platforms) he
might well have said: "Yes that's the sort of thing I was struggling to identify
in 1944".

To put it
crudely, an engineer, familiar with heat engines
only, will, after inspecting the construction of an
electric motor, be prepared to find it working
along principles which he does not yet
understand. He finds the copper familiar to him
in kettles used here in the form of long, wires
wound in coils; the iron familiar to him in levers
and bars and steam cylinders here filling the
interior of those coils of copper wire. He will be
convinced that it is the same copper and the same
iron, subject to the same laws of Nature, and he
is right in that. The difference in construction is
enough to prepare him for an entirely different
way of functioning. He will not suspect that an
electric motor is driven by a ghost because it is
set spinning by the turn of a switch, without
boiler and steam. .......

To be expanded, showing how something with the apparent regularity and precision
of clockwork mechanisms can continue operating for long periods of time in
accordance with principles of Quantum mechanics but not in accordance with the
kinds of reliable regularities found in statistical mechanics arising out of
numerosity of individuals.

So, despite QM being famous for its "uncertainty principle" and for replacing
the determinism of Newtonian mechanics with pervasive non-determinism, it is
only QM, not Newtonian mechanics, that can explain the kind of persistence and
replication of structure that is required for the existence of living things in
all their many forms, including the ability to absorb, store, and use "negative
entropy" either extracted from solar radiation (using photosynthesis) or by
consuming and digesting parts of other organisms that have acquired such stores.

Schrödinger's little book provides a profound example of the
importance for science of theories that attempt to answer the (Kantian) question
"How is X possible?" Sloman (2014).

Note added 6 Mar 2016
It seems that recent work by Jeremy England referenced below,
can be seen as extending the ideas in What is life? by using Quantum
theory to explain how it is possible for some important precursors of life to
come into existence on a lifeless planet. (Thanks to Aviv Keren for drawing my
attention to this.) Some of the structures that might spontaneously form could
be building blocks not only for some of the earliest forms of life (as described
by Ganti (1971/2003)) but possibly also for some of
the "construction-kits" and forms of scaffolding required for biological
evolution. See
http://www.cs.bham.ac.uk/research/projects/cogaff/misc/construction-kits.html